pm=int(input("请输入PM2.5的值："))
if 0<pm<51:
    print("优")
elif 51<pm<100:
    print("良")
elif 101<pm<150:
    print("轻度污染")
elif 151<pm<200:
    print("中度污染")
elif 201<pm<300:
    print("重度污染")
else:
    print("严重污染")